Multiple: a number that can be divided by another number without a remainder.
We only need to find one side (1 - 25), as we can just double it for the negative. Here are the positive multiples of 6 from 1 to 25:
6, 12, 18, 24 <----------- 4 multiples
Double that, and we got both positive and negative.
2(4) = 8
Remeber, 0 is also a multiple!
8 + 1 = 9
:)